New fuel assemblies for light water reactors are shipped from the fabrication plant to their destination in a generally horizontal position in special containers. Some fuel assembly designs require protection against shipping related fretting damage to the fuel rods caused by the fuel rods moving within spacer grids which are used to provide lateral bracing and spacing, and which maintain the fuel rods in fixed positions during reactor operations. Since even the slightest repeated relative movement between contacted fuel rods and spacers can result in fretting of the fuel cladding, minimization of such relative movement is desired during fuel shipment as well as during reactor operations.
In a nuclear reactor, the reactor core contains nuclear fuel which is typically in the form of fuel rods grouped together in fuel assemblies. Groups of fuel assemblies are arranged into a matrix to form a core capable of controlled fission reaction. Each fuel rod is typically a long member approximately 0.4 inches in diameter and 8-15 feet long containing fuel usually in the form of a stack of fuel pellets which are surrounded by tubular cladding. The fuel rods which make up an assembly are grouped together to form a plurality of longitudinally extending members which are supported vertically by two parallel end plates, an upper and a lower tie plate. These plates are usually connected to one another by tie rods or other structural elements.
A fuel assembly or bundle may also include other non-fuel bearing members. Examples include guide tubes in pressurized water reactors to form passageways for control rods which assist in controlling the rate of fission, instrumentation tubes for in-core instrumentation, spacer capture rods, and water rods or water channels which modify the neutron moderation in the assembly by permitting coolant moderator to flow from the lower tie plate through or into the upper part of the fuel assembly and out of the upper tie plate. The spaces between adjacent fuel rods and between fuel rods and non-fuel bearing members create flow channels through which coolant and/or moderator can circulate.
The fuel assembly, whether in a pressurized water reactor, boiling water reactor, high temperature gas cooled reactor, or any other type of reactor, functions in part to maintain the fuel rods in a fixed position, ideally free of vibration and restrained from bowing or other movement during normal and other reactor operating conditions. Spacers or spacer grids which provide rod-to-rod spacing and lateral bracing are typically designed to allow differential axial expansion of the fuel rods. Spacers and spacer components are therefore generally not rigidly connected to the fuel rods. Spacers are comprised of spacer cells each of which includes a combination of fixed contact points and/or resilient springs to maintain the desired rod-to-rod spacing and to maintain the fuel rods in their proper lateral positions. The springs and dimples which necessarily contact the fuel cladding also permit some radial movement of the fuel rod which can therefore cause fretting.
The springs in the spacers or spacer grids are designed to meet a number of design criteria which effect the maximum forces they may exert on the fuel rod at the beginning of service life. Maximum loads are dictated by considerations such as local stresses in the cladding tube and damage to fuel rods during insertion into the fuel assembly. It is not always practical, however, to satisfy service requirements, while at the same time accommodating very different types and kinds of loads that are incurred during shipment of the fuel assembly. Typically, loads resulting from the shipment of fuel assemblies include multi-directional acceleration of the fuel rods.
Some fuel assemblies have fuel rods and non-fuel bearing members arrayed such that the gap or distance between each of the fuel rods or members is constant. One way to protect such fuel assemblies during shipment is to insert shims between all of the fuel rods during packaging. These shims generally have a single thickness which corresponds to the nominal gap between the rows of fuel rods or members.
However, some fuel assembly designs do not necessarily have the same distance between fuel rods and/or between fuel rods and non-fuel bearing members of the fuel assembly. Such fuel assemblies may have: fuel rods of different outside diameters; fuel rods in rectangular, triangular or hexagonal arrays; variable fuel rod to fuel rod distances; variable fuel rod to inner water channel distances; and/or variable fuel rod to water tubes or water rod distances. In addition, the distance between fuel rods or fuel rods and other non-fuel bearing members at one elevation in the fuel assembly can vary from that at a different elevation due to axial variation of the fuel rod diameter. Furthermore, the use of part length fuel rods also affects the distance between fuel rods at different elevations within the fuel assembly.
Although the inclusion of one or more of such design features in the fuel assembly optimize one or more performance features of the fuel design, the protection of the fuel rods against fretting during shipment becomes particularly complex, if not infeasible, as well as costly and time consuming.
It would thus be an advantage over prior art designs to provide a shipping shim which can accommodate multiple variable sized gaps between fuel rods and/or between fuel rods and non-fuel bearing components of a nuclear fuel assembly.